Antigen and Antibody Detection
Serology helps in the investigation of the ongoing pandemic especially
in cases where NAAT assays are found to be negative and the link between
illness (clinical manifestations) and COVID-19 is very strong, so that
samples can be collected for both acute and convalescent phases for
serology (WHO, 2020).
In response to SARS coronavirus infections, different types of
immunoglobulins (antibodies) including IgG and IgM are produced and can
change in level in the course of infection. The antibodies can be
undetectable at the initial infection stage; IgG can be detected even
after the illness has been resolved. The antibody tests for the disease
include ELISA to detect both IgG and IgM antibodies which is more
reliable especially 21 days of infection, and Immunofluorescence assay
to detect only IgM or IgG antibodies after about 10 days of infection.
Positive antibody test signifies infection whereas the reverse indicates
no infection has taken place (WHO, 2003). Immunoblot is another
serologic technique for SARS-CoV antibodies (Maschinen et al., 2005).
Serologic techniques for identification of COVID-19 antibodies including
IgA, IgG and IgM from clinical specimen such as ELISA are less reliable
than molecular tests and can potentially be utilised for early
diagnosis. There is limitation around the onset of the symptoms when
incubation and transmission are high, and the body may not likely start
producing antibodies. The response of antibodies to the viruses usually
takes many days or weeks to be clearly detected, and, negative outcome
do not count out infection particularly at early stage (Cheng et al,
2020; USFDA, 2020). Cross reactivity with antibody to non COVID-19
proteins must also be put into consideration, so that positive result
may be as a result of recent or past infection with other coronaviruses.
Serology is more relevant in a situation whereby patient present with
complication of late disease when RT-PCR is likely to produce false
negative result as a result of dropping of viral load over time (Cheng
et al, 2020).
Detection of SARS-CoV directly using ELISA have not been possible,
rather, its nucleocapsid protein from respiratory specimen, faeces and
urine; potential nucleocapsid protein has been used as reliable
diagnostic tool to detect the virus (Lau et al., 2004). ELISA was known
to be highly specific, less expensive and labour intensive than RT-PCR.
Western blotting (WB) and IF assay have also been used to detect serum
SARS-CoV (Leung et al., 2004). Detection of influenza virus antigen by
IF directly from clinical sample have been available and is less
complex, providing results at point of care but have suboptimal
sensitivity to rule out disease, this challenge may exist probably in
the case of COVID-19, therefore implementation of test of this nature
need clear guidance on correct interpretation.
It was already recommended that negative serologic results do not rule
out SARS-CoV-2 infection, rather molecular testing should be conducted
particularly on samples of highly suspected individuals, furthermore,
the results of antibody testing should not be relied on for confirmation
or exclusion of COVID-19 infection or its status as positive outcome may
be as a result of present or fast encounter with other non COVID-19
coronavirus strains (USFDA, 2020). Monoclonal antibodies against
nucleocapsid of 2019-nCoV is currently generated for future antigen
detection test (Cheng et al., 2020). Recombinant spikes protein
(S-protein) and nucleocapsid protein (N-protein) are currently in use to
manufacture diagnostic kit for 2019-nCoV serum antibodies (Xu, 2020).